As shown in FIG. 1, gas turbine engines 10 of the type commonly found on many aircraft include a compressor 20, a combuster 30 and a turbine 40. The compressor 20 compresses air which is then mixed with fuel for combustor 30 to ignite. The combustor 30 exhausts gases turn the vanes of the turbine(s) 40. Power from the rotating turbine 40 operates the compressor 20.
Turbine engine compressors 20 can be designed to supply more compressed air than is needed to operate the engine 10. This additional compressed air from the compressor 20 can be used for tasks other than feeding the combustor 30. For example, it is common to bleed some of the compressed air from the compressor 30 and route it to other equipment onboard the aircraft such as de-icers, cabin pressurization systems and the like.
Each of the aircraft engines 10 can be used as a source for compressed bleed air. It is generally desirable to balance the amount of bleed air obtained from each pair of engines 10 to equalize wear and other engine operating conditions. Various techniques have been designed in the past to control such bleed air balancing.
An exemplary prior art airflow balancing control 100 for a dual engine, dual bleed aircraft is shown in FIG. 2. In this prior art exemplary design, hot bleed air flow from the compressor 30 of a first engine 10a is regulated by a first pressure regulating shutoff valve 110a, and hot bleed air flow from the compressor 20 of a second engine 10b is regulated by a second pressure regulating shutoff valve 110b. These two streams of regulated hot bleed air are provided to precoolers 112a, 112b that receive cold air from the fan or prop (in the case of a turboprop) on the front of the engines 10a, 10b. The amount of cold air is also regulated by valves 114a, 114b. The outputs of precoolers 112a, 112b are provided to opposite input ports 116a,116b of a T-configuration bleed air manifold 118. Upon entering the input ports 116a, 116b, the bleed air flow encounters temperature sensors 108a, 108b respectively that measure the bleed air temperature. The two different bleed air streams are then passed through respective venturis 120a, 120b before being combined into a common air supply to aircraft systems available at a manifold output port 122. Differential pressure transducers 104a, 104b placed across each venturi 120a, 120b measures the bleed air flow pressure differential across the venturi's throat. Pressure transducers 106a, 106b placed within the manifold 116 before the bleed air flow encounters the venturis 120 measures the absolute pressure of each engine's bleed air flow.
It can be observed that digital controller 102 receives the signals provided by the two differential pressure transducers 104a, 104b, the two pressure transducers 106a, 106b and the two temperature sensors 108a, 108b. Digital controller 102 processes these signals to determine the mass air flow of each bleed air stream. Control law software operating on the controller 102 calculates currents that are delivered to modulate the PRSOV 1 (Pressure Regulating Shutoff Valve) 110a and PRSOV 2 (Pressure Regulating Shutoff Valve) 110b butterflies that independently control how much air to bleed from the compressors 20 of each of engines 10a, 10b, respectively. In this way, the controller 102 can dynamically balance the mass air flow from the respective bleed air streams to ensure that each engine 10a, 10b contributes exactly half of the total bleed air pressure provided at manifold output port 122.
Generally speaking, the exemplary illustrative non-limiting controller 102 may implement a control law algorithmic process that processes these currents as follows. The pressure delivered to the aircraft systems should stabilize in the desired set point in acceptable settling time; the respective pressure overshoot and undershoot should also be acceptable and under steady state condition; each engine should contribute half of the total air bleed flow.
While much work has been done in the past, further improvements are possible and desirable. In particular, it would be highly advantageous to simplify the air bleed balancing control system described above to reduce the number of pressure transducers, to reduce system weight and to optimize the bleed balancing control system.
An exemplary illustrative non-limiting method of controlling bleed air flow may comprise measuring the differential pressure between a first bleed air flow and a second bleed air flow; generating a differential pressure correction signal in response to said measured differential pressure; measuring the pressure of a bleed air flow obtained by combining said first and second bleed air flows; and controlling valves modulating said first and second bleed air flows based at least in part on said measured pressure and said differential pressure correction signal.
The method may further include measuring the temperature of said first bleed air flow and measuring the temperature of said second bleed air flow, and said controlling includes modulating said first and second bleed air flows in response to said measured temperatures. The differential pressure measuring may comprise measuring the differential pressure between the throat of a first venturi through which said first bleed air flow passes, and the throat of a second venturi through which said second bleed air flow passed. The differential pressure measuring may comprise measuring the differential pressure between two different regions of a manifold used to combine said first and second bleed air flows. The method may further include deriving said first bleed air flow from a first gas turbine engine, and deriving said second bleed air flow from a second gas turbine engine. The generating may includes applying a proportional gain, an integration and a differentiation. The controlling may comprise controlling first and second pressure regulating shutoff valves.
A dual engine bleed airflow regulator may comprise a single pressure transducer that measures the pressure of combined first bleed air flow from a first engine and a second bleed air flow from a second engine; a single bi-directional differential pressure transducer that measures the difference between the pressure of said first bleed air flow and said second bleed air flow; and a controller responsive to said single differential pressure transducer and said single pressure transducer, said controller generating a first control signal for modulating said first bleed air flow and generating a second control signal for modulating said second bleed air flow.
Non-limiting exemplary illustrative features and advantages include:                Reduced number of pressure transducers.        Reduced number of differential pressure transducers.        Lower weight.        Less expensive system.        Acceptable differential pressure transducer drift identification and compensation by the digital controller during the system power-up (when there is zero pressure)        Better total system MTBF (mean time before failure) and, consequently, it requires less maintenance tasks.        